The invention describes a method for making a heterogeneous catalyst and, more particularly, to a method for making a metal heterogeneous catalyst with monolayer (ML) or sub-ML metal thickness.
Catalyst technology plays a critical role in the production of materials related to many facets of the world economy including petroleum refining, pharmaceutical productions, chemical processing and production, and environmental cleanup. Heterogeneous catalytic reactions are widely used and are commonly characterized by reactions performed with the reactant(s) and product(s) in the fluid or gas phase and the catalyst in the solid phase. In heterogeneous catalytic reactions, the reaction occurs at the interface between phases; that is, the interface between the fluid or gas phase of the reactant(s) and product(s) and the solid phase of the supported catalyst. Hence, the properties of the surface of a heterogeneous supported catalyst are significant factors in the effective use of that catalyst. Specifically, the surface area of the active catalyst, as supported typically on a metal oxide substrate, and the accessibility of that surface area to reactant chemisorption and product desorption are important. These factors affect the activity of the catalyst, defined by the rate of conversion of reactants to products. The chemical purity of the catalyst and the catalyst support have important effects on the selectivity of the catalyst, which is the degree to which the catalyst produces one product from among several products, and the lifetime of the catalyst.
Most heterogeneous catalysts are composed of a selected combination of active material, promoter and support. Catalysts with high surface areas are desirable to reduce the cost of material and to increase the activity of the catalyst (that is, the product production rate per unit weight). Active catalytic materials for most non-biological chemical reactions come generally from the transition metals of the Periodic Table and are generally considered to include vanadium, iron, cobalt, nickel, molybdenum, ruthenium, rhodium, palladium, cadmium, tungsten, rhenium, osmium, iridium, platinum and mercury, although other metals are catalytic in specific reactions. Promoters are included to increase activity and stability and use compounds, particularly oxides, which include elements selected from lithium, sodium, magnesium, boron, potassium, calcium, barium, lanthanum, cerium, and thorium. The supports are typically chosen from materials that have high surface areas and may or may not contain sites active for catalyzing specific reactions and generally include Group IIa and IIIa elements, alkaline earth and transition metal oxides (such as Al2O3, SiO2, TiO2, and MgO, zeolites and activated carbon). Low surface area supports, or substrates, can also be used. These substrates include ceramics, such as the high temperature form of alumina, or metallic monoliths.
Generally, catalytic activity is proportional to catalyst active surface area. Therefore, a high specific area is desirable. That surface area must be accessible to reactants and products as well as to heat flow. The chemisorption of a reactant by a catalyst surface may be preceded by the diffusion of that reactant through the internal structure of the catalyst. To minimize cost and maximize catalytic activity, it would be desirable for metal catalytic materials to be deposited on supports at levels of sub-MLs to a few MLs in thickness. When the catalytic material is a metal and the support is an oxide, the metal deposited on the oxide surface generally does not wet the surface because the metal-to-oxide interaction is relatively weak. At room temperature, metal islands form on an oxide. Because it is known that reactions often occur at step sites, these islands, having many edges and steps, are desirable in this sense, but all interior metal material is in effect wasted. Thus a reduction in island size is highly desirable.
The morphology of the resulting metal layer is thus quite variable within several nanometers (nm) of the oxide surface, and the resulting interface is typically quite non-uniform and weak. To strengthen the interface, reactive metals are sometimes added, such as brazing compounds. However, it is sometimes undesirable for such metals to be added because they often react with the oxide to form an intermediate layer, which is poorly defined and contrary to the desired catalytic structure. Additionally, the shape of the particles on the support surface depends on the interfacial energy between the metal and the oxide, and can in principle vary from flat two-dimensional islands (if strong interactions are present) to three-dimensional amorphous or faceted objects having minimal contact to the oxide (if the interaction is very weak). The ability to control this shape by interfacial engineering of the adhesion energy would provide an additional tool for catalyst design.
Useful would be a method for producing a catalyst with maximally dispersed and controlled ML or sub-ML amounts of active material on a support in order to produce a more effective and more cost efficient catalytic material.